samples: drivers: watchdog stm32 wwdg decrease apb clock

The most of the wwdg peripherals requires a reduced apb clock
to pass the sample.drivers.watchdog.stm32_wwdg testcase.
This apb1 prescaler is added to the overlay file
for that particular clock.
Specific overlay for the stm32h7 family because of the APB1
bus clock named d2ppre1.

Signed-off-by: Francois Ramu <francois.ramu@st.com>